`zcash_client_backend`: Support import of seed phrases from transparent-only wallets.

[nuttycom]

More specifically, we realized as part of the discussion that there is probably an existing bug related to the import of seeds from transparent-only wallets, where we are not at present scanning any of the internal key tree (which is where Bitcoin wallets regularly send change) so if you import a seed that was previously used with a transparent-only wallet, you're likely to be missing funds.

Then again, if you do that, then you're also not doing gap limit scanning at the moment, so both of these should be addressed at once. I'll open an issue for that and it can get stuck in the backlog somewhere.

Originally posted by @nuttycom in https://github.com/zcash/librustzcash/pull/1185#discussion_r1488643222